Method for transferring and stacking of semiconductor devices

ABSTRACT

A method is presented in which an active element, e.g. a semiconductor device, is embedded in a passive circuitry formed on a low-cost substrate, having good dielectric properties. After forming the active element on a first substrate, the active elements are singulated and transferred to a second substrate. The active element is bonded to this second substrate and the portion of the first substrate, on which this active element is created, is removed selectively to the active element and the low-cost substrate. On this second substrate passive circuitry may be present or it can be formed after the attachment of the active element. The passive circuitry is interconnected to the active element or other components or dies present on the low-cost substrate.

RELATED APPLICATION INFORMATION

This application claims claims priority under 35 U.S.C. §119(e) to U.S.Provisional Application Serial No. 60/178,994, filed Jan. 28 ^(th) 2000,which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor devices, in particular tomicro- or millimeterwave devices, embedded in an interconnect platformand manufacturing methods thereof.

2. Background of the Invention

The increasing usage of MMIC's (Monolithic Millimeterwave IntegratedCircuit) in application fields such as the automotive industry is astrong driving force to develop alternative technologies with equalperformance level but at lower cost. In standard MMIC technology theactive element and the passive circuitry are formed in a monolithic wayon a single substrate. This substrate must fulfil all the requirementswith respect to e.g. the growth of semiconductor layers, high frequencyperformance, manufacturability and cost. An alternative technology isthe hybrid integration of individual HEMT's (High Electron MobilityTransistor) with passive circuitry on low-cost substrates. In this way,the epitaxial area consumption per chip can be reduced dramatically.

In U.S. Pat. No. 5,675,295, hereby incorporated by reference, amicrowave oscillator device for a receiver or a transmitter isdescribed. This oscillator device comprises a high frequency oscillatingcircuit including an active device. The active device, i.e. a verticaldiode, is formed on an undoped, semi-insulating, GaAs substrate. Thismanufacturing method comprises the steps of depositing a sacrificiallayer on this GaAs substrate, followed by the deposition on thissacrificial layer and subsequent patterning of the layers, e.g.semiconductor layers, which compose the active device. An example of amanufacturing method of such active elements can be found in “W-bandhigh-gain amplifier using InP dual-gate HEMT technology”, by K. van derZanden et al, in Proc. InP and related Materials, 1pp7, pp249-252,hereby incorporated by reference. The thus formed vertical active device(see FIG. 1b) is then separated from the undoped GaAs substrate e.g. byapplying the epitaxial lift-off (ELO) technique wherein the sacrificiallayer, sandwiched between the active device and the semi-insulatingsubstrate, is selectively etched. After separation the active device istransferred to and attached on a second substrate. This second substratecan be any other substrate, comprising passive circuitry andinterconnects.

SUMMARY OF THE INVENTION

One aspect of the invention is directed to providing high frequencydevices comprising at least one semiconductor element interconnected toa passive circuitry. Thus, a preferred embodiment provides a method offabricating a semiconductor device, comprising depositing two or morelayers of semiconducting material onto a first substrate to form a firstsemiconductor stack, singulating said first semiconductor stack to forma first group of singulated semiconductor stacks, wherein said firstgroup is comprised of at least a first singulated semiconductor stackhaving a top semiconducting layer, a bottom first substrate layer, andan inner semiconducting layer in contact with said bottom firstsubstrate layer, providing a second substrate having a first conductivelayer and a first bonding material deposited thereon, selecting saidfirst singulated semiconductor stack from said first group, attachingsaid upper semiconducting layer of said first singulated semiconductorstack to said conductive layer to form a second semiconductor stack, andremoving said two or more layers of semiconducting material in saidsecond semiconductor stack from said first substrate layer to therebyform a semiconducting device in which said inner semiconducting layer isexposed. In a further preferred embodiment, a tandem cell cell isprovided by employing as the second substrate a semiconductor having aband gap that is different from the band gap of said secondsemiconductor stack.

In another preferred embodiment, a method fo fabricating a semiconductordevice is provided, comprising depositing a sacrificial layer onto afirst substrate, depositing two or more layers of semiconductingmaterial onto said sacrificial layer to form a first semiconductorstack, singulating said first semiconductor stack to form a first groupof singulated semiconductor stacks, wherein said first group iscomprised of at least a first singulated semiconductor stack having atop semiconducting layer, a bottom first substrate layer, and an innersemiconducting layer in contact with said bottom first substrate layer,providing a second substrate having a first conductive layer and a firstbonding material deposited thereon, selecting said first singulatedsemiconductor stack from said first group, attaching said uppersemiconducting layer of said first singulated semiconductor stack tosaid conductive layer to form a second semiconductor stack, and removingsaid two or more layers of semiconducting material in said secondsemiconductor stack from said first substrate layer to thereby form asemiconducting device in which said inner semiconducting layer isexposed.

Another aspect of the invention is directed to providing a manufacturingmethod for hybrid integration of individual semiconductor devices withpassive circuitry. A preferred embodiment thus provides a method offabricating a hybrid device comprised of an optical waveguide and asemiconductor device, comprising depositing two or more layers ofsemiconducting material onto a first substrate to form a firstsemiconductor stack, singulating said first semiconductor stack to forma plurality of singulated semiconductor stacks, each having an topsemiconducting layer and a bottom first substrate layer, providing asecond substrate having an optical waveguide deposited thereon,attaching said upper semiconducting layer of said singulatedsemiconductor stack to said optical waveguide to form a hybrid stack,and removing said two or more layers of semiconducting material in saidsecond hybrid stack from said first substrate layer.

The present invention may provide the advantage of an easy transfer ofsingle semiconductor devices from their original substrate to a secondsubstrate. The proposed process of transferring the semiconductordevices offers an improved handling and alignment towards the secondsubstrate of the devices. The proposed transfer method is very robust.

The present invention can provide the advantage of an improved stackingof semiconductor slices to obtain tandem solar cells.

The present invention can provide the advantage that, during the processof hybrid integration, the active side of the semiconductor element isprotected and remains essentially unaffected.

The present invention can provide the advantage of re-using the originalsubstrate after the active device is transferred to a second substrate.

The present invention provides an easy and highly accurate substrateremoval, which is a large advantage for subsequent processing, and stillmaintains a high level of performance. In combination with the limitedenvironmental load compared to As, this makes Ge for these specificapplications a more attractive substrate material than GaAs.

The present invention combines the advantages of a good growth substratewith a high performance active microwave circuit.

The present invention can offer the advantage of combining MCM-Dtechnology and HEMT technology yielding both high frequency devices andoptical devices on the same substrate. This substrate comprises theoptical waveguide forming an optical interconnect between differentcomponents or circuits or parts thereof present on this substrate.

The present invention can provide the advantage of an improved andeasier stacking of semiconductor slices to obtain tandem solar cells.

DESCRIPTION OF THE DRAWINGS

In relation to the appended drawings the present invention is describedin detail. All drawings are intended to illustrate some aspects andembodiments of the present invention. Devices and fabrication steps aredepicted in a simplified way for reason of clarity. Not all alternativesand options are shown and therefore the invention is not limited to thecontent of the given drawings. It will be apparent to a person skilledin the art that there are several other equivalent embodiments or otherways of executing the present invention, the spirit and scope of thepresent invention being limited only by the terms of the appendedclaims.

FIGS. 1a and 1 b: prior art cross section of a metamorphic HEMTstructure grown on a Ge-substrate. a: cross section through device stackafter deposition, b: cross section through device after removal of theGe substrate and buffer layer

FIG. 2: prior art schematic representation of an MCM-D technology

FIGS. 3a-e: method of embedding an active device, e.g. HEMT, in aninterconnect technology, e.g. MCM-D, comprising passive devicesaccording to one embodiment of the present invention.

FIGS. 4a-d: method of embedding an active device, e.g. HEMT, in aninterconnect technology, e.g. MCM-D, comprising passive devicesaccording to another embodiment of the present invention.

FIG. 5: contacting of the transistors and construction of the MCM-Dpassive microwave circuit.

FIG. 6: schematic representation of system build-up for a short rangeradar system, illustrating the industrial application of the presentinvention.

FIG. 7: Schematic cross section of a microwave patch antenna in MCM-Dtechnology, illustrating the industrial application of the presentinvention.

FIG. 8: integration of components, processed in the same or differenttechnology, on a MCM (-D) substrate, illustrating an embodiment of thepresent invention

FIG. 9: cross section of a photodetector HEMT and an optical waveguideillustrating an embodiment of the present invention

FIG. 10: formation of an optical waveguide in a stack of BCB layers andfurther embedding of a photodetector HEMT according to an embodiment ofthe present invention

FIG. 11: schematic cross section of a tandem solar cell according to oneembodiment of the invention

FIG. 12: electrical equivalent scheme of the cross section given in FIG.11.

DETAILED DESCRIPTION

In relation to the appended drawings the present invention is describedin detail in the sequel. It is apparent however that a person skilled inthe art can imagine several other equivalent embodiments or other waysof executing the present invention, the spirit and scope of the presentinvention being limited only by the terms of the appended claims.

FIG. 1a gives a schematic overview of an active device, in this examplea metamorphic HEMT structure with a graded InAlAs buffer and a doubledoped heterostructure, grown on a Ge-substrate. Such a stack ofsemiconductor layers can be grown on a variety of substrates such as Ge,GaAS, InP. Preferably Ge is used as the substrate as such substrate isvery well applicable for growing InGaAs/InAlAs High Electron MobilityTransistors. Ge substrates are more advantageous than GaAs or InPsubstrates as these Ge substrates can be larger, are less fragile andless expensive. The high dielectric loss of Ge substrates for combiningpassive circuitry and active devices has a number of disadvantages: e.g.the cut-off frequency f_(T) of the transistors drops to 45 GHz whereascomparable devices formed on GaAs or InP substrates can have a cut-offfrequency f_(T) of 90 GHz or higher. Moreover the losses of thetransmission lines in the microwave circuits formed on Ge substratesbecome very large.

Various low-cost substrates can be used as a second substrate. A verysuitable and known technology platform is the so-called MCM-Dtechnology, which is also used in U.S. Pat. No. 5,675,295. MCM-D (MultiChip Module-Dielectric) is a thin film technology in which alternatingthin layers of insulating and conductive materials are deposited on alow-cost substrate, such as glass or sapphire. The metal lines in oneconductive layer run perpendicular to the metal layers in anotherconductive layer. In this MCM-D technology, fixed passive components,such as resistors or capacitors, can also be formed simultaneously,while on top of the MCM-stack chips can be die-bonded by means of solderbump or flip chips techniques. Openings, i.e. vias, in these insulatinglayers are created to interconnect the passive components, the chips andthe wiring layers. Compared to the standard MMIC technology, MCM-Dtechnology offers a substrate with lower dielectric losses at lowercost. The cross section given in FIG. 2 shows an example of suchlow-cost substrate technology. This MCM-D technology consists of abuild-up of 3 metal layers (15-16, 17, 18) embedded in BCB (12, 13), adielectric material with low dielectric constant (∈_(r)=2.7) and lowdielectric losses (loss angle tanδ=8 10⁻⁴). The complete structure isbuilt on a low dielectric loss glass substrate (9). During the built-upof the stack of conductive and dielectric layers, passive components areformed. This metal-dielectric multilayer stack comprises TaN resistors(14), Ta₂O₅ capacitors (11-10), inductors and distributed microwavecomponents. TaN resistors are mainly used as high-frequency 50 ohmterminations. Ta capacitors are used for realising large capacitors. Forsmall capacitors a parallel plate BCB capacitor can be used. Both theTaN resistors and the Ta capacitors can be used for realizing a stablebiasing of active circuitry.

In a first aspect of the invention the integration of ultra-thinsemiconductor devices with passive circuitry on a low-cost substratewith good dielectric properties is disclosed. The semiconductor devicecomprises a stack of layers, e.g. semiconductor layers, grown on firstsubstrate. After forming the semiconductor device this first substrateis singulated and the dies are transferred to a second substrate. Afterremoval of the first substrate the attached dies are interconnected onthis second substrate with passive elements, thereby employing ainterconnect technology. Finally an active microwave circuit isobtained.

In a preferred embodiment of this first aspect a metamorphic HEMT grownon a Ge-substrate is embedded in an MCM-D interconnect platform. Thisembodiment uses backside contacting of the HEMTs to the MCM-D lines, asillustrated in FIGS. 3(a-e). This technique combines the advantages of aless expensive, good growth substrate with a high performance activemicrowave circuit.

The Ge substrate measures 50×50 mm² and has a thickness of 200 μm.Although highly Ge resistive substrates are available, a Ge substrate(1) having a 50 ohm cm resistivity is chosen from a cost point of view,releasing the constraints on material purity. Since the Germanium onlyacts as a sacrificial substrate, as will be later on explained, itsconductivity is not significant for the performance of the final deviceor circuit. The MBE (Molecular Beam Epitaxy) grown layer structure isdepicted in FIG. 1a and is very similar to what can be grown on GaAs.After an initial GaAs nucleation layer (2) the buffer (3) is graded fromAlAs to In_(0.57)Al_(0.43)As, followed by an inverse step toIn_(0.52)Al_(0.48)As to form a stress relaxation layer (4). The doubleSi δ-doped structure (upper doping level is 5·10¹² cm⁻² and the lower is2.5·10¹² cm⁻²) is grown lattice matched on this virtual substrate. Thesubstrate is called virtual as the lattice constant of GaAs, grown onthe Ge substrate, is transformed into the lattice constant of InP troughthe relative thick buffer layer (3). The HEMT active layers are formedas if these layers were grown on a full InP substrate. This stack ofHEMT active layers comprises a buffer (5 a), a spacer (5 b), the channel(6), followed by spacer (7 b) and a Schottky layer (7 a). Finally acapping layer of a Si-doped In_(0.53)Ga_(0.47)As layer (8) for ohmiccontact formation is deposited. The active area of the HEMT device isobtained by wet mesa etching, deposition and alloying of Ni/Au/Ge ohmiccontacts (20: 20 a, 20 b) and Cr/Au c-gate metal deposition (20 c). TheGe substrate is singulated to yield individual devices or array ofdevices. As contact levels are applied in this step of the processing,some devices can be interconnected into an array. FIG. 1b shows a crosssection of the HEMT device after removal of the Ge substrate and thebuffer layer. The contact pads to source (20 a) and drain (20 b) arefree and can be contacted from the backside. In the prior art, e.g. U.S.Pat. No. 5,675,295, the device, as shown in FIG. 1b, would be ready fortransfer to another substrate.

Although Ge can be used as substrate material e.g. for the growth ofmetamorphic devices with a high level of performance and a DC behaviorcomparable to GaAs-based devices, its less likely to use Ge as substratefor the construction of performant RF circuits. Because the cut-offfrequency f_(T) and the maximum oscillation frequency f_(max) for the Gebased devices (respectively 45 and 68 GHz) are low compared to similarGaAs based devices (f_(T)=90 and f_(max)=130 GHz), there is a need forto remove the Ge-substrate to expel capacitive parasitics originatingfrom the conductive Ge. In this preferred embodiment an MCM-D substrateis used, e.g. glass, as such substrate has a low-cost and gooddielectric characteristics. The main disadvantage of such technologyplatform is the absence of active material e.g. to construct highfrequency components such as microwave circuits. To realise completemicrowave circuits active components are added to the passive microwaveMCM-D by using flip chip.

The InAlAs/InGaAs HEMTs are integrated in the MCM-D stack by by flipchip connecting the HEMT (21) on the glass substrate (9), as illustratedby FIG. 3a. After device formation and singulation of the Ge-substrateinto single HEMT devices, the HEMT devices (21) on top of theGe-substrate (1) are transferred to the MCM-D substrate (9). As the,relative thick, Ge-substrate (9) is still present during this transferthe handling of the HEMT devices becomes very easy and the transferprocess is more suitable for industrial application.

A thin non-cured BCB layer (22) is used for the adhesion between theglass substrate (9) and the HEMT (21). Preferably a thin layer, e.g. 1μm or less, of BCB is deposited by spin-coating on top of the substrate(9). The HEMT device (21) is upward-down or flip chip connected to thisadhesion layer (22). The BCB layer is then cured to improve the adhesionfurther (FIG. 3a). A BCB layer is used as adhesion layer as it iscompatible with the other materials used in the MCM-D technology.

After attaching the stack of the HEMT device and the Ge-substrate to theMCM-D substrate the Ge substrate (1) is next removed in a CF₄-O₂ plasmaetch (see FIG. 3b). A highly selective etch process has been developed,based on CF₄ and O₂ plasma Reactive Ion Etching (RIE), being astraightforward method compared to GaAs thinning. The settings of thisselective etching process are: CF₄ flow is 100 sccm, O₂ flow is 10 sccmand the pressure is 300 mTorr. The etching of the Ge-substrates takesplace at 30° C. with 50 Watt RF input power. The etch rate of Getypically is 3.5 μm/min, resulting in an etch time of about 60 minutesto remove the complete 200 μm thick substrate, leaving only the 2 μmthick epitaxial layer. Approximately 200 μm Ge is etched in 70 min, witha selectivity to the GaAs buffer layer (2) of about 1/100. The Ge can bereused by capturing the Ge from the reaction gass flow present at theexhaust of the RIE process chamber. The recycled Ge can be used to grownagain Ge-substrates. As can be noticed in FIG. 4 the BCB layer, notcovered by the HEMT stack, is also etched during the RIE step, but theglass substrate remains essentially unaffected. The GaAs buffer (2) isremoved by a non-selective H₂SO₄—H₂O₂ solution. This wet etching stepthins the HEMT device, including the contact pads, to about 3-5 μm,preferably less then 3 μm, and leads to the structure of FIG. 3b. Onlythe InGaAs/InAlAs HEMT layers (3-8) and the Au contacts (20) remain.Such a thin structure can be integrated in the MCM-D stack withoutdetoriating the planarising properties of the MCM-D substrate and allowsthe further processing of the MCM-D stack and the fixation of chips ontop of this MCM-D substrate. The step height created by the embedding ofthe transferred HEMT device is smaller than the thickness of the appliedBCB layers.

The following steps are the realisations of the Ta and TaN structures onthe MCM-D, e.g. used to form the resistor (14) and capacitors (10-11).The bottom Al layer (15) covers the HEMT structure and protects itduring these processing steps (FIG. 3c).

After patterning the Ta and TaN structures a photoresist layer (notshown in FIG. 3d) is deposited and patterned to cover the Ta/TaNstructures (14, 10) and the active area (23) of the transistor. By usinga phosphoric acid solution only the active area of the transistorremains and the contacts (20: 20 a, 20 b), initially formed on the topof the HEMT stack before their transfer to the second substrate (9),become free (FIG. 3d). The last steps include the stacking andpatterning of the BCB (12,13) and copper layers (17,18) for therealisation of the passive structures. The copper metallisation (17,18-19) is also used for contacting the HEMT. This gives finally theactive microwave MCM-D structure (FIG. 3e). The Ni/Au component layer(19) is formed and allows the bonding of chips or other components onthe MCM-D stack. Chips can be attached to further process signals fromand to the active microwave MCM-D structure.

In a second embodiment of this first aspect a metamorphic HEMT grown ona Ge-substrate is embedded in an MCM-D interconnect platform afterforming resistors and capacitors. This second embodiment uses anoptimized flip chip type of interconnection with Indium used as abonding material, rather then as bump material, as illustrated in FIGS.4(a-d) and 5.

An alternative integration of InAlAs/InGaAs HEMTs in MCM-D is realisedby doing flip chip of the HEMT on the glass substrate using Indium (In)as a bonding material. This approach is illustrated in FIGS. 4(a-d) and5.

First the TaN (14) and Ta layers (10-11) are realised using a standardMCM-D process. This process also realises the Al bottom contacts (25)for the HEMT device (FIG. 4a). A thin In layer (26) is evaporated andpatterned on Al (FIG. 4b). The thin, e.g. 300 nm or less, In layer isused as mechanical and electrical interconnection layer between theMCM-D substrate and the HEMT. The HEMT (21) is put on the In bondinglayer (26) by the flip chip bonder and subsequently heated to 150° C.(FIG. 4c). The Ge substrate (1) is next removed in a CF₄—O₂ plasma etchselective to the GaAs layer (2) as disclosed previously. The GaAs bufferis removed by a non-selective H₂SO₄—H₂O₂ solution as disclosedpreviously. The HEMT device is hence again thinned to about 3-5 μm orless and leads to the structure of FIG. 4d. The last steps include thestacking and patterning of the BCB (12,13) and copper layers (17,18-19)for the realisation of the passive structures. The copper metallisationis also used for contacting the HEMT. This gives finally the activemicrowave MCM-D structure shown in FIG. 5.

The hybrid integration as illustrated above can be used for a largenumber of applications in the microwave and even millimeter range. As anexample the integration of two HEMT devices is given to obtain ashort-range radar system which can be used in collision avoidancesystems. In FIG. 6 an outline is given of a short range radar systembased on frequency modulation techniques. An oscillator (29) can betuned in frequency and sends a frequency modulated signal to the antennathat radiates in the front direction. The radiation is reflected at ametallic object like a car and arrives back at a second antenna. Thisreceived signal is mixed (30) with the sent signal to generate a lowfrequency beat signal which frequency is proportional to the distanceand the incoming speed of the object. To generate a complete system inMCM-D the MCM-D technology is extended with a back-side processing stepto generate a patch antenna (17) inside the MCM-D layout (FIG. 7). Suchpatch antennas can be used to radiate the frequency modulate signal orto detect the reflected signal. For the construction of the dopplerradar in principle one HEMT is sufficient when one uses aself-oscillating mixer concept. In such structure this single HEMT isused for generating the microwave oscillation and the down conversion ofthe reflected signal at the same time. This circuit has however atrade-off between output power of the oscillator and noise generation inthe mixer. This effectively limits the range of the radar to a couple ofmeters. By using two transistors, one for the oscillator (29) and onefor the mixer (30), the biasing and matching of both devices can beoptimized to get sufficient oscillator output power with a reasonablenoise figure for the mixer. This is the topology given in FIG. 6. Allmicrowave circuits, e.g. matching circuits, biasing circuits andantennas are realized in the MCM-D technology. The two HEMT devices canbe integrated by flip chip according to one of the options outlinedabove. The quasi-monolithic technique according to one of the aboveoptions, by embedding and back-contacting the HEMTs, should eliminatethe parasitic effects of the interconnection as much as possible andshould result in much better results especially at the higher millimeterfrequencies.

In a second aspect of the invention an optical interconnect system onthe low-cost substrate is disclosed. The semiconductor devices grown onthe first substrate can be transferred to the second substrate in whichan optical waveguide is formed. The semiconductor devices are used as aoptical receiver/transmitter to convert electrical signals into opticalsignals and vice versa.

As disclosed in previous embodiments HEMT devices can be integrated orembedded into a circuit which is defined on the MCM substrate or withinthe composing layers. The MCM technologies, e.g. MCM-D, offer a platformto combine different technologies, such as III-V or BiCMOS (31),allowing the integration of devices, each having optimized propertieswith respect to data processing and operation frequency, on a singlesubstrate as shown in FIG. 8. As stated before, chips can be attached tothe MCM-D stack to further process signals from and to the activemicrowave MCM-D structure, comprising the HEMT device. The exchange ofdata between the components in or on an MCM-D stack is normally done byelectrical signals. If however this data transmission could be done bylight signals, very high data transmission rates could be obtained. Inorder to perform such communication, “light channels” or “lightwaveguides” and “transducing devices” are required. The “light channels”will transport the light from one “transducing device” to another. The“transducing device” will transduce or convert the light signal receivedfrom the “light channel” into an electrical signal that can be furtherprocessed by normal electronic circuitry.

The “light channels” are created by forming a trench of a first material(core layer) in a substrate of a second material (cladding layer). Bothmaterials have different optical characteristics, such as theirrefractive index n. A schematic cross-section of the operation of suchlight channel is given in FIG. 9. The light being transported in thefirst material (core) (30), will not be encapsulated completely withinthis first material but some part of the Electro-Magnetic (EM) wave (32:dashed circles), as light is an EM-signal, will extend outside thistrench, in the cladding layer (13). This exponentially decaying part ofthe light wave is called the “evanescent field”, as it is locatedoutside the core of the light waveguide. The BCB (Benzo-Cyclo-Butene)layers (12, 13), used to electrically isolate the conductive layers inthe MCM-D metal-dielectric stack, are available in different variants,each variant having different optical properties. An optical waveguideor light waveguide can be created making use of the difference inoptical characteristics of the different variants of BCB. An overview ofthe optical properties of BCB is given in “Fabrication andCharacterization of Singlemode Polymeric Optical Waveguides”, by DanielSchneider, Ph. D. dissertation university of Trondheim, Norway 1998. BCBcan be used as a first material (core: 30), using the 4022 variant, andcan be used as a second material (cladding: 13), using the 3022 variant.The first variant, 4022, is similar to a photosensitive resin andtherefore can be exposed to light and being wet developed afterwards.The second variant 3022 is not light sensitive and has to be patternedusing standard exposing and etching techniques. Normally this 3022variant is used as dielectric layer in the MCM stack. The pattering ofthe BCB layers is done when via's, e.g. 33, are created to contactunderlying metal levels. At this level or stage in the MCM-D processingsimultaneously a trench can be formed outlining the optical waveguide.After removal of the resist used during this patterning step, the trenchcan be filled with the optical variant BCB4022 . The filling can e.g. bedone by spin-coating of the BCB material, followed by an etch-back ofthe BCB only leaving the trench filled with BCB4022 . As BCB4022 hasresin like properties, BCB4022 can be spin-coated on top of the bottompart (13 a) of the cladding layer (13), exposed to light using a maskand wet developed to have only the optical waveguide (30) lineremaining. Afterwards the remaining part (13 b) of the cladding layer isdeposited and the overall structure is planarized.

In the example illustrated in FIG. 8, the photoHEMT is placed on top ofthe MCM-stack and the optical waveguide is formed in the top BCB layer(13). Another approach is illustrated in FIG. 10. On top of the MCM-Dsubstrate (9) the optical waveguide is created by first forming thestack of the cladding layer (34) comprising the optical waveguide (30).On top of this stack processing can continue as illustrated by FIGS.3a-e.

A HEMT device has photosensitive properties, as for example the currentbetween drain and source contacts of the HEMT can be modulated to aminor extent by incident light. No specific processing is thereforerequired to manufacture dedicated light transducers as such “lightsensing transistor” is established simultaneously with and is equivalentto the other HEMT transistors, used e.g. in mixers (29) ordown-converters (30) as explained above. Only the interconnection tosuch HEMT used as light-sensing device is different compared to a HEMTused as electronic device, but this only requires design effort and nochange in processing technology. Both high frequency devices and opticaldevices can be formed on the same substrate, preferably MCM-D substrate,comprising the optical waveguide.

If a HEMT device is processed and transferred in an hybrid way on top ofthe MCM-D stack in a way similar to other embodiments of the presentinvention, the gate of the device (20 c) can be located right on top ofsuch “light channel” as shown in FIG. 9. As can be seen in this figuresome of the light will be penetrating in the heterolayers (5 a-8 ) ofthe devices and affect e.g. the gate leakage current. This penetratedlight will be absorbed in these semiconductor layers and createselectron-hole pairs. A HEMT device consists of III-V compoundsemiconductor material.

In a second aspect of the invention the stacking of ultra-thinsemiconductor devices to form tandem solar cells is disclosed.

This embodiment is a further illustration of the first embodiment. Ifsemiconductor devices are grown on a first, sacrificial substrate, andthen, after singulating, transferred to a second substrate, one can usethis technique to obtain a stack of semiconductor slices. In tandemsolar cells slices of semiconductor material having different band gapsare stacked. As each slice comprise a semiconductor or compoundsemiconductor material with a band gap different from the materialspresent in the other slices, each slice will be sensitive to aparticular range of electro-magnetic radiation. Whereas standard siliconsolar cells are only susceptible for light having energy quanta largerthan the bandgap of silicon (1.1 ev), a tandem solar cell e.g.comprising a slice of silicon and a slice of GaAs can detect not onlylight in 1.1 eV wavelength range but also in larger wavelengths as thebandgap of GaAs is smaller (about 0.92 ev). The overall efficiency ofsuch tandem solar cell is higher then of the single solar cell. Thepresent invention allows the stacking of several slices of semiconductormaterial. FIG. 11 illustrates this embodiment. Both opposite sides ofthe semiconductor slices (40, 41) are contacted by a set ofperpendicular metal wiring (44, 43) to contact opposite sides of thejunctions formed in these semiconductor slices. The connect these twoslices, each slice forming a p-n junction in series, the metal level inone direction at a given level is contacted with the metal wiring in thesame direction at a higher level. The metal wiring (44) is contacted byopenings (45) in the insulating layer (42). The metal wiring (43) iscontacted by openings (46) in the insulating layer (42). The insulatinglayer electrically isolates the p-n junctions and the wiring. FIG. 12gives the electrical equivalent scheme of the structure given in FIG.11. It clearly illustrates the series connection of the p-n junctions.The stack given in FIG. 11 can be formed using other embodiments of thepresent inventions. The semiconductor slice (40) can be grown on anothersubstrate to yield the desired p-n junction. This substrate can besingulated into a slice having the desired dimensions. The device can betransferred to a second substrate (46) and attached hereto, e.g. byusing a BCB adhesion layer. As disclosed in other embodiments the firstsubstrate on which this semiconductor slice was formed will be removed,e.g. during a dry etch processing step. Metal wiring can be formedbefore or after attaching the die (40). A second BCB layer is depositedon top of the first metal wiring (44) and on top of the slice (40). Asecond metal wiring (43) in a direction substantially perpendicular tothe direction of the first metal wiring. This second metal wiringcontacts the opposite side of the p-n junction formed in thesemiconductor slice. Again a layer of BCB is deposited to electricallyisolate the different levels of metal wiring and to act as a glue layerto fix a second slice (41) transferred to this stack. The processingsequence disclosed above can be repeated. Openings are formed in thedielectric layers (42) to connect the appropriate levels of metal wiringto obtain e.g. a series or a parallel connection of the devices.

What is claimed is:
 1. A method of fabricating a semiconductor device,comprising depositing two or more layers of semiconducting material ontoa first substrate so as to form a first semiconductor stack; singulatingsaid first semiconductor stack to form a first group of singulatedsemiconductor stacks, wherein said first group is comprised of at leasta first singulated semiconductor stack having an upper semiconductinglayer, a lower first substrate layer, and an inner semiconducting layerin contact with said lower first substrate layer; providing a secondsubstrate having a first conductive layer and a first bonding materialdeposited thereon; selecting said first singulated semiconductor stackfrom said first group; attaching said upper semiconducting layer of saidfirst singulated semiconductor stack to said first conductive layer ofsaid second substrate so as to form a second semiconductor stack; andremoving said first substrate layer and one or more layers of said twoor more layers of semiconducting material from said second semiconductorstack so as to form a semiconducting device wherein said innersemiconducting layer is exposed.
 2. The method of claim 1, wherein saidsecond substrate is glass.
 3. The method of claim 1, wherein said secondsubstrate is a multi chip module dielectric stack.
 4. The method ofclaim 1, wherein said first bonding material is an organic polymer. 5.The method of claim 1, wherein said first singulated semiconductor stackis a high electron mobility transistor.
 6. The method of claim 1,wherein said removing further comprises selectively etching said firstsubstrate.
 7. A method of fabricating a hybrid device comprised of anoptical waveguide and a semiconductor device, said method comprising:depositing two or more layers of a semiconducting material onto a firstsubstrate so as to form a first semiconductor stack; singulating saidfirst semiconductor stack to form a plurality of singulatedsemiconductor stacks, each having an upper semiconducting layer and alower first substrate layer; providing a second substrate having anoptical waveguide deposited thereon; attaching said upper semiconductinglayer of said singulated semiconductor stack to said optical waveguideto form a hybrid stack; and removing said first substrate layer and oneor more layers of said two or more layers of said semiconductingmaterial from said hybrid stack.
 8. The method of claim 7, wherein saidsecond substrate is a multi chip module dielectric stack.
 9. The methodof claim 7, wherein said singulated semiconductor stack is a highelectron mobility transistor.
 10. The method of claim 7, wherein saidremoving comprises selectively etching said first substrate.
 11. Amethod of fabricating a semiconductor device, comprising: depositing asacrificial layer onto a first substrate; depositing two or more layersof semiconducting material onto said sacrificial layer to form a firstsemiconductor stack; singulating said first semiconductor stack to forma first group of singulated semiconductor stacks, wherein said firstgroup is comprised of at least a first singulated semiconductor stackhaving an upper semiconducting layer, a lower first substrate layer, andan inner semiconducting layer in contact with said bottom firstsubstrate layer; providing a second substrate having a first conductivelayer and a first bonding material deposited thereon; selecting saidfirst singulated semiconductor stack from said first group; attachingsaid upper semiconducting layer of said first singulated semiconductorstack to said conductive layer to form a second semiconductor stack; andremoving said first substrate layer and one or more layers of said twoor more layers of semiconducting material from said second semiconductorstack so as to form a semiconducting device wherein said innersemiconducting layer is exposed.
 12. The method of claim 11, whereinsaid second substrate is glass.
 13. The method of claim 11, wherein saidsecond substrate is a multi chip module dielectric stack.
 14. The methodof claim 11, wherein said first bonding material is an organic polymer.15. The method of claim 11, wherein said first singulated semiconductorstack is a high electron mobility transistor.
 16. The method of claim11, wherein said removing further comprises selectively etching saidsacrificial layer.
 17. A method of producing a plurality ofsemiconductor devices, comprising: forming a stack of semiconductorlayers on a first substrate; singulating said stack of semiconductorlayers and said first substrate, so as to obtain a plurality ofsub-parts; attaching at least one of said sub-parts to a secondsubstrate, wherein said second substrate is in contact with saidsingulated stack of semiconductor layers; removing from each of saidsub-parts attached to said second substrate a singulated portion of saidfirst substrate so as to form a semiconductor device on said secondsubstrate.
 18. The method of claim 17, further comprising formingresistors and capacitors on said second substrate.
 19. The method ofclaim 17, further comprising forming interconnects on said secondsubstrate.
 20. The method of claim 17, wherein said second substrate isa multi-chip module dielectric stack.
 21. The method of claim 17,wherein said semiconductor device formed on said second substrate is ahigh electron mobility transistor.
 22. The method of claim 17, whereinsaid semiconductor device formed on said second substrate is an activemicrowave circuit.
 23. The method of claim 17, wherein said firstsubstrate is germanium substrate.
 24. The method of claim 23, furthercomprising removing said germanium substrate in a CF4-O2 plasma etch.25. The method of claim 17, further comprising forming an opticalwaveguide in said semiconducting device.
 26. The method of claim 17,further comprising: forming an interconnect pattern and dielectricisolation; forming a second semiconductor device on top of said firstsemiconductor device according to the method of claim
 17. 27. The methodof claim 1, wherein said second substrate is a semiconductor having aband gap different from that of said second semiconductor stack.